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Full text of "Text Book Of Mechanical Engineering"

Zeuner Diagrams for Link Motion.

665

G K, CK. Then, with certain limitations, the radii vector of CK
Mail shew closing to steam of the Meyer valve for respective
positions of engine crank.

Let the back valve be adjusted to any desired width, and R be
measured when at mid position; with radius R describe the circle
L M N o. Strike the steam lap at P R Q ; the vectors within p T s u
then shew opening to steam for the respective crank positions.
Admission is given at p by main valve, in the usual manner;
after u the opening is also controlled by the R circle, and when
the difference vector equals R, as at cs, cut-offtakes place. We
see from this diagram how decrease of R secures an early cut-off
and vice vers^ and rapidity of cut-off can be judged by decrease
towards s of the vectors of the shaded area., The exhaust circle
is governed by the main valve only. (See Appendix //.,/. 895.)

Zeuner Diagrams for Stephenson Link Motion.-
KLnowing that decreased travel when linking-up causes earlier
cut-off, we may now examine, by Zeuner diagram, Fig. 656, the
exact result obtained. Taking the upper diagram:

Open rods. With throw as radius, strike travel circle F E, and
draw valve circle D E for full gear. Draw the link A B, represented
by full travel of die, with D A, D B as the distance between die and
sheave centres. Through point G, where D A 'and valve circle
intersect, draw EGH to meet the centre line DC in H. Then DH
is the diameter of the valve circle in mid gear, and any other
circle, as D <?, will have its diameter bounded by E H ; position <?,
between E and H, corresponding to proportionate position between
A and c The centres K L j will form a parabola, lying concave
to D and with vertex at j. Draw the lap circle ab, and ^ erect
perpendicular */Y. YE will.be the lead in full gear, and the
amount of lead will increase as the travel decreases, shewn
by the shaded figure, being dn in mid position, or equal to
the lap.

Crossed rods. In the lower diagram, the full-gear circle o P is
set out &9 before, also the link M N. The crossing point R, made
by the further rod MQ, is joined to P, when SP bounds the
diameters of valve circles. The centres of the circles now make
a parabola conyex to o, and with vertex at T. Strike the lap
circle /& and draw the perpendicular ^ x. x p is the lead in full